Well if you have another with MG, the whole flock would probably test positive for MG. I would treat this hen with the same antibiotic for MG symptoms. The stress of the attack and pecking may have brought on symptoms, who knows, but it is worth treating her. Is there not a disinfectant cream in the UK that can be bought for treating the wounds on her face (not in her eyes, of course?) Sudocream and there is another popular cream in France called Savulon that have chlorhexidene which might be good for the wounds. The eyes would need Neosporin, Bacitracin, or Polysporin,  or a similar ointment, but that would be okay to put on the wounds as well.